Even on a night which is too dark for diurnal vision, invisible infrared light is richly provided by the stars. Human vision cannot utilize this infrared night time light from the stars because the so-called near-infrared portion of the spectrum is invisible for humans. A night vision device of the light amplification type can provide a visible image replicating the night time scene. Such night vision devices generally include an objective lens which focuses invisible infrared light from the night time scene onto the transparent light-receiving face of an I.sup.2 T. At its opposite image-face, the image intensifier tube provides an image in visible yellow-green phosphorescent light, which is then presented to a user of the device via an eye piece lens.
A contemporary night vision device will generally use an I.sup.2 T with a photocathode behind the light-receiving face of the tube. The photocathode is responsive to photons of infrared light to liberate photoelectrons. These photoelectrons are moved by an electrostatic field to a microchannel plate (MCP) having a great multitude of dynodes, or microchannels, with an interior surface substantially defined by a material having a high coefficient of secondary electron emissivity. The photoelectrons entering the microchannels cause a cascade of secondary emission electrons to move along the microchannels so that a spatial output pattern of electrons which replicate an input pattern, and at a considerably higher electron density than the input pattern results. This pattern of electrons is moved from the microchannel plate to a phosphorescent screen by another electrostatic field to produce a visible image.
A power supply for the I.sup.2 T provides the electrostatic field potentials referred to above, and also provides a field and current flow to the microchannel plate(s). Conventional night vision devices (i.e., since the 1970's and to the present day) provide automatic brightness control (ABC) and bright source protection (BSP). ABC maintains the brightness of the image provided to the user substantially constant despite changes in the brightness (in infrared and the near-infrared portion of the spectrum) of the scene being viewed. BSP prevents the I.sup.2 T from being damaged by an excessively high current level in the event that a bright source, such as a flare or fire, comes into the field of view.
The current drawn by the MCP varies as a function of the average brightness of the image. The output impedance of the power supply for most MCPs is also quite high. As a consequence, the voltage applied to the MCP will vary as a function of average image intensity, unless the voltage to the MCP is regulated.
The voltage delivered to the MCP is also often adjusted to obtain a desired amount of ABC and BSP. For bright image intensities, for example, it is often desirable to reduce the voltage to the MCP.
The conventional method of developing the high voltage needed to drive a MCP has been to generate an oscillating signal from the electricity supplied by a DC battery, to feed that oscillating signal into a step-up transformer, and to direct the output of the transformer into a voltage-multiplying circuit. The output of the voltage-multiplying circuit is then delivered to the MCP.
The conventional method of regulating the voltage to the MCP has been to sense the voltage on the MCP, to generate an error if the voltage on the MCP is other than as desired, and to direct that error into a circuit which adjusts the amplitude of the high voltage AC waveform that is rectified and multiplied by the multiplier to correct for that error.
This conventional approach, however, has several drawbacks. One drawback is that it usually requires the use of a separate transformer to generate and control the amplitude of the voltage delivered to the MCP. Because the power supply for the typical night vision device must also generate and control voltages for other segments of the system, this generally requires the power supply to have several transformers. This multiplicity of transformers increases costs, reduces reliability, and reduces overall power supply efficiency.
Another problem with this type of prior art feedback system is that the reactance, in the transformer often causes the response of the system to be slow and unstable.